Thermoplastic aliphatic polyamides are often referred to as Nylon. Nylons are typically condensation copolymers formed by reacting a diamine and a dicarboxylic acid or ring-opening polymers formed by polymerization of lactames, such as aminocaproic acid. One of the most common variants is nylon 66, also known as PA 66, which name refers to the fact that the diamine (hexamethylene diamine) and the diacid (adipic acid) each donate 6 carbons to the polymer chain.
Nylon was developed as a synthetic replacement for silk and substituted for it in many different products, such as parachutes, after silk became scarce during World War II. Nylon fibers are today used in many applications, including fabrics, carpets, musical strings, and rope. Solid or reinforced nylon (engineering polymer) is used for mechanical parts such as machine parts, gears, containers, tubes, primary and secondary design elements and other low- to medium-stress components previously cast in metal. Engineering-grade nylon is processed by extrusion, casting, and/or injection molding.
In order to improve the mechanical strength, aromatic polyamides, such as aramid, have been developed. Furthermore, aromatic polyamides are less prone to absorb water than aliphatic polyamides. Absorption of water will affect the mechanical strength negatively. However, the processability of aromatic polyamides is inferior to one of aliphatic polyamides. Further, aromatic polyamides are more brittle and less resistance to chemical solvents compared to aliphatic polyamides.
It would thus be desirable to be able to use aliphatic polyamides in applications wherein aromatic polyamides typically are used.
There have been attempts in the art to improve the mechanical strength of the polyimides, which are related to aromatic polyamides.
U.S. Pat. No. 5,493,002 discloses oligoimides endcapped with PEPA (Phenylethynyl phtalic anhydride). The PEPA endcapped oligoimides are cured, i.e. cross-linked, at about 400° C. Similarly, U.S. Pat. No. 5,066,771 discloses use of EPA (ethynyl phtalic anhydride) as an endcapper for oligoimide. The disclosed EPA endcapped oligoimides was cured, i.e. cross-linked, in a step wise manner including heating at 200° C. for 4 hours, at 250° C. for 2 hours, at 290° C. for 1 hour and lastly at 320° C. for 6 hours.
Further, there have been attempts in the art to improve the mechanical strength of the aromatic polyamides. EP 1 988 114 discloses wholly aromatic polyetheramides endcapped with PEPA. Wholly aromatic polyamides are thermo stable and withstands the heat required to cure the cross-linkable end-capper PEPA.
However, as well known within the art, aliphatic polyamides, such as various types of nylon, are less thermo stable and would degrade at temperatures typically used to cross-link PEPA. Thus, cross-linking of PEPA in polyamides would require catalysis or long term cross-linking at lower temperatures. Accordingly, PEPA has not find use as cross-linkable end-capper for aliphatic polyamides.
As an alternative to PEPA, also ethynyl phtalic anhydride (EPA) has been used as cross-linker in polyimides (cf. Hergenrother, P. M., “Acetylene-terminated Imide Oligomers and Polymers Therefrom”, Polymer Preprints, Am. Chem. Soc., Vol. 21 (1), p. 81-83, 1980).
Although polyimides comprising EPA may be cross-linked at a lower temperature, i.e. at about 250° C., it suffers from other drawbacks. The exchange of the phenyl ethynyl group to an ethynyl group implies that other reaction pathways than the desired curing mechanism, such as chain extension, are favored. As a consequence, EPA has not found any wide use as a replacement to PEPA as a low temperature curing end-capper. Further, the manufacture of EPA requires protective group chemistry hampering its commercial potential.
Neither EPA is suitable as end-capper for polyamides. In addition to the drawback mentioned above, cross-linking of EPA will be initiated at temperature below the normal processing temperature, typically 290 to 310° C., of nylon 66, thus limiting its possible use as a cross-linker for nylon 66 end-capped with EPA would, at least to certain extent, cross-link during processing.
Polyamic acids, and their corresponding polyimides, endcapped with PEPA or EPA have been suggested for use in various applications in the art. As an example, JP 2010186134 discloses a photosensitive resin containing an optical base generator (A) and polyamic acid (B), wherein the polyamic acid (B) may have terminal polymerizable group(s). The terminal polymerizable groups are selected from polymerizable groups known in the art, such as anilines or dianhydrides comprising carbon-carbon double or triple bonds. Specifically disclosed examples of polymerizable end-cappers include maleic anhydride, 4-aminocinnamic acid, 4-ethynylaniline, 3a,4,7,7a-tetrahydroisobenzofuran-1,3-dione, 3a,4,7,7a-tetrahydro-4,7-methanoisobenzofuran-1,3-dione, 3a,4,7,7a-tetrahydro-4,7-epoxyisobenzofuran-1,3-dione, EPA and PEPA.
According to Wollf et al (cf. Synthesis, 2007 (5), 761-765) N-phenylphthalimides with carbon substituents in the 3-position, such as (4-(1-octyn-1-yl)-2-phenyl-1H-Isoindole-1,3(2H)-dione, 4-(1-hexyn-1-yl)-2-phenyl-1H-Isoindole-1,3(2H)-dione, and 4-(3,3-dimethyl-1-butyn-1-yl)-2-phenyl-1H-Isoindole-1,3(2H)-dione, are accessible by Sonogashira coupling reaction of the corresponding bromo derivatives. 3-alkyl substituted N-phenylphthalimides may be used as synthetic intermediates for the production of pre-organized hydrogen bonding donors for the synthesis of supramolecular affinity molecules.
U.S. Pat. No. 6,344,523 addresses the disadvantageous of the too high curing temperature of PEPA discussed above and discloses that use of sulfur or organic sulfur derivatives as curing promoters may lower the curing temperature of phenylethynyl terminated imide oligomers. However, the introduction of such promotors suffers from other disadvantages. In particular the curing results in chain extension rather than cross-linking as two ethynyl groups react along with one sulfur radical ultimately forming a thiophene structure.
Thus, there is need within the art for an alternative cross-linking monomer, overcoming the above-mentioned deficiencies, to be used as cross-linking monomer for aliphatic polyamides, such as PA66.